Clipping is the limiting of the amplitude of an output signal when an input signal is above a given amplitude. Typically clipping occurs when the amplitude of the output signal reaches one of the supply rails, or an output stage driving transistor reaches its maximum output current by going into saturation or reaching maximum drive current.
For many applications, especially audio applications, it is desirable that clipping be performed symmetrically. Asymmetrical clipping introduces even harmonics to the output signal. Even harmonics do not naturally occur in audio signals and their presence can produce noticeable distortion for a listener.
Where clipping occurs as a result of the output signal amplitude reaching a supply rail at a different amplitude, with respect to the normal bias point of the output stage, from the amplitude at which the driving transistor reaches its maximum output current asymmetrical clipping will occur.
As a result, most output stages for applications where symmetrical clipping is desirable set the output stage bias point to be midway between one supply rail and the output voltage when the driving amplifier reaches its maximum output current. Typically, an output stage will consist of a bipolar junction transistor (BJT) amplifier whose emitter is connected to one supply rail and whose collector is connected through a resistor to the other supply rail. The input to the output stage is a current at the base of the transistor, while the output of the output stage is at the collector of the transistor. The only factor limiting upward signal excursion, assuming the other supply rail is a positive supply rail, is the difference between the bias point at the output and the positive supply rail. This difference is easily controlled. On the other hand the factors limiting downward signal excursion are the current gain of the amplifier, the driving current at the base of the amplifier and the saturation point of the amplifier. These factors are processing sensitive and interdependent. In order to obtain substantially symmetrical clipping each factor is set up at the time of processing and cannot be adjusted thereafter. In order to change the amount of clipping it is necessary to have separately processed circuits with each of the factors set for each desired clipping level.